1. Field of the Invention
The present invention relates to the field of computer chip manufacturing. In particular, the invention relates to the design of an improved wire bonding surface and method for wire bonding for use with semiconductor devices using compliant dielectric materials.
2. Prior Art
Conventional Semiconductor devices are formed by the selective deposition of numerous materials including silicon dioxide (SiO.sub.2) dielectric layers and conductive layers upon silicon wafers. The uppermost conductive layers are typically made of Aluminum. The silicon wafers are then sawed to form individual computer chips. The top of each chip typically includes a number of exposed metal regions, known as bonding pads. The connection to packaging and to external devices is accomplished by bonding leads to these bonding pads. These bonding pads are typically formed of aluminum. Conventional bonding methods include the use of thermosonic or ultrasonic bonding methods. Thermosonic bonding is typically used to bond leads having gold wires while ultrasonic is typically used with leads having aluminum wires.
Polyimides are increasingly being studied for forming inter-layer dielectric layers as well as for final passivation of integrated circuits. Though polyimide layers are easy to process and have good planarization, these layers are highly compliant. When conventional bonding pad structures and automated wire bonding processes are used with semiconductor devices having polyimide dielectric layers, serious reliability problems and yield reductions occur. Poor wire pad strength and unacceptable levels of failures occur over a wide range of machine parameters. These failures are primarily due to the compliant nature of the polyimide which does not allow for efficient coupling of the ultrasonic energy at the interface between the lead and the bonding pad surface. The ultrasonic energy is dissipated within the polyimide layers instead of into the bonding process.
What is needed is a bonding pad structure and a method of bonding for forming a high yield, low failure semiconductor chip for use with compliant dielectric materials, and in particular polyimide. This structure and method should use conventional materials for forming bonding pads and should allow for the use of conventional bonding methods and machinery.